The present invention relates to an ink jet recording method for recording an image on a recording medium by using an ink jet recording head.
In an ink jet recording system, an ejection energy generator such as a piezoelectric element or an electric heat converting element, etc. is used as an ejection driving source, and an ink housed in an ink chamber is ejected through ink ejecting orifices made in an orifice plate arranged within the ink chamber so as to allow the ejected ink to be attached to a recording medium such as a recording paper sheet or a film. The ink attached to the recording medium is absorbed by and fixed to the recording medium so as to perform the image recording. In general, a nonvolatile and highly hygroscopic solvent, i.e., a nonvolatile wetting agent, is added to the ink used in such a recording system to make the ink unlikely to be dried so as to prevent the ink in the ink ejecting orifice from being dried and from bringing about plugging of the ink ejecting orifice. The ink having a wetting agent added thereto is certainly effective for preventing the plugging of the ink ejecting orifice. However, the ink ejected onto the recording medium is unlikely to be dried promptly.
In performing an image recording such as a full color image recording in an ink jet recording system, a special recording paper sheet is used in many cases in order to allow the ink to be attached to, absorbed by and fixed to the recording medium satisfactorily. To be more specific, the special recording paper sheet is prepared by coating the surface of a mother material of an original paper sheet with a dispersion prepared by dispersing pigments such as SiO.sub.2. CaO or an organic material in a hydrophilic binder. However, the special recording paper sheet of this type is costly, making it difficult to use the special recording paper sheets freely.
In general, ordinary paper sheets or high quality paper sheets are widely used as the ink jet recording paper sheet. However, these paper sheets are not satisfactory in the capability of absorbing the ink, with the result that problems given below take place in the case of applying an image recording to the ordinary paper sheets or high quality paper sheets that are widely used in general.
Specifically, if an image recording is performed by using an ink relatively satisfactory in image quality of the characters and line images and low in drying speed, it is possible for the ink droplets recorded on the recording medium to be present on or inside the paper sheet under the state that the ink droplets are not sufficiently dried and fixed. In this case, the ink droplets are not dried promptly, with the result that the coloring matter such as a dye or a pigment within the ink droplets is fluidized and diffused so as to cause the ink droplets positioned contiguous to each other or superposed one upon the other to be mixed with each other. It follows that a color blurring called bleed is generated between adjacent ink droplets of different colors so as to deteriorate the image quality.
It may be possible to suppress the color blurring problem between different colors in a full color image by using as the recording medium ordinary paper sheets or high quality paper sheets that are not satisfactory in the ink absorption capability and by using an ink that is relatively high in drying capability and permeability. In this case, however, the ink droplets promptly permeate deep into the recording medium and, thus, the coloring matter does not remain on the surface of the paper sheet. As a result, the printing region of the ink droplets is caused to have a low density, leading to a narrow range of the color reproducibility. In addition, since the ink is diffused toward the surface of the recording medium so as to be dried, the line tends to be thickened and the image quality is greatly deteriorated by the feathering.
The ink jet recording method disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 7-47762, is known to overcome the problems pointed out above. Specifically, disclosed in this prior art is an ink jet recording apparatus provided with a plurality of recording heads for ejecting ink droplets differing from each other and using an aqueous ink having a water-soluble dye dissolved therein. It is taught that the recording is performed by using a black ink and color inks, i.e., inks of yellow, magenta and cyan in general, in combination. The black ink is specified to have an absorption coefficient Ka in the recording medium, i.e., an ink absorption coefficient determined by Bristol method, of 0.5 (ml/m.sup.2.multidot.ms.sup.1/2) or less and a wetting time Tw of 50 to 200 ms. On the other hand, each of the color inks is specified to have the absorption coefficient Ka in the recording medium of 1.0 (ml/ m.sup.2.multidot.ms.sup.1/2) or more and the wetting time Tw of 20 ms or less.
To be more specific, the printing is performed first by using an ink having the absorption coefficient Ka of 0.5 (ml/m.sup.2.multidot.ms.sup.1/2) or less and the wetting time of 50 to 200 ms. Then, after the minimum delay time Td determined by the absorption characteristics of the ink, the amount of the ink droplet, and the number of ink droplets per unit area, the printing is performed by using an ink having the absorption coefficient Ka of 1.0 (ml/ m.sup.2.multidot.ms.sup.1/2) or more and the wetting time Tw of 20 ms or less. It is taught that the particular printing technique makes it possible to achieve printing of sharp characters and line images and to suppress the color blurring on the ordinary paper sheet that is a recording medium low in the ink absorption rate.
However, it is difficult to determine the optimum printing conditions by simply referring to the absorption coefficient Ka derived from the result of the Bristol method for measuring the ink permeability into the recording medium, the wetting time Tw, the coarseness index, the specification of the printer, the printed pattern image, etc. In order to determine the compatibility and compatible conditions between the ink and the recording medium, it is necessary to evaluate the ink permeability into the recording medium by the Bristol method, to evaluate the printed state by using an evaluating tester, and to evaluate the printing test by a machine manufactured on the trial basis in all the combinations. It follows that much time and labor are required, leading to an inefficient determination of the compatibility and compatible conditions between the ink and the recording medium.
It should also be noted that, if the absorption coefficient Ka is small in the case of operating the printer at a high printing speed, the fixation is rendered unsatisfactory. Therefore, where colors of, for example, yellow, magenta and cyan are superposed one upon the other in the order mentioned in performing a color image recording, it is possible for the magenta ink droplets to cover the yellow ink droplets ejected first while the yellow ink droplets are in the process of permeating the recording medium so as to bring about the bleeding problem. It is also possible for the next ink droplets to be ejected before the previous ink droplets are fixed. Also, if the absorption coefficient is unduly large, color blurring is brought about in general. Particularly, the feathering, which is a blurring of a single color, tends to become excessively prominent.
It should also be noted that, if magenta ink droplets are ejected onto the adjacent dot positions while the yellow ink droplets ejected previously are in the process of permeating the recording medium, the magenta ink droplets are brought into contact with the yellow ink droplets so as to bring about the bleeding problem. Also, the dot image is rendered large so as to impair the resolution of the printed image. The bleeding does not take place in the monochromatic printing. However, a problem remains unsolved that the printed dot is enlarged by the bleeding.
What should also be noted is that it is necessary to prepare at least two kinds of inks differing from each other in properties for a single printer, giving rise to the problems that it is troublesome to prepare the solvents and that the ink cost is increased. Further, if the properties of the ink are changed, it is unavoidable to change the compatibility of the ink to the head, making it necessary to change the driving conditions for ejecting inks having different properties from the head. In the worst case, it is necessary to change the construction of the head.